1. Field of the Invention
The present invention relates to an optical fiber cable, more particularly relates to an optical fiber cable using tape cores.
2. Description of the Related Art
Optical fiber tape cores are tape-like assemblies of a plurality of optical fibers, for example, four, arranged in parallel and joined integrally by plastic. Such optical fiber tape cores can take any shape in a completely free space, as the optical fibers comprising them are seldomly locally bent and therefore there is little increase of transmission loss in the optical fibers due to bending.
When a plurality of optical fiber tape cores are placed in a closed space, however, the movements of the tape cores occurring along with changes in the closed space interference with each other. Some tape cores are subjected to unreasonable bending and therefore the transmission loss in the optical fibers sometimes increases.
Therefore, in conventional optical fiber cables using tape cores, two or more tape cores are stacked to form assemblies so as to make the directions of movement of the tape cores substantially the same and prevent the movement of only part of the tape cores from being obstructed.
As a method of arranging such a tape core stack in a cable, there is known the method of arranging the tape core stack in grooves of a red-shaped member.
In this method, a plurality of grooves are formed along the longitudinal direction in the surface of a plastic (resin) rod-shaped member having a circular cross-section. The cores of the tape core stack are inserted into those grooves and then the entire assembly is sheathed to complete the optical fiber cable.
Since such an optical fiber cable uses a grooved rod-shaped member, however, there are limits to how far the outer diameter of the cable can be reduced. Further, the manufacturing cost becomes higher.
As another means for preventing obstruction of movement of tape cores, there is known the loose tube structural of optical fiber cables. The xe2x80x9cloose tube structurexe2x80x9d is a structure where the optical fiber cores or tape cores are placed in a tube made of polybutylene terephthalate, propylene, or another plastic (resin) and then the spaces in the tube are filled with a jelly or other gel-like filler.
Loose tube structure cables are shown in FIG. 1 and FIG. 2.
In the cable of the loose tube structure illustrated in FIG. 1, the tape core stack 57 is placed in a plastic pipe-shaped hollow member 56. The pipe-shaped hollow member 56 is either left as it is or the pipe-shaped hollow member 56 is filled with a jelly-like substance 58. Next, a plurality of these are wound around a center tension-bearing member 55 and the entire assembly is covered by a sheath 59.
The cable of the loose tube structure illustrated in FIG. 2 has a central core type structure having a loose tube portion at the center of the cable in which a stack 61 of the tape cores is accommodated. In this central core structure optical fiber cable, the tape core stack 61 is accommodated in the loose tube which is then filled with a jelly 68 and then covered with a sheath 63 along with tension-bearing members 64, 64. By suitably adjusting the viscosity of the jelly 68, it is possible to have the tape cores 61 move in the tube and therefore have the strain applied to the optical fibers eased even if the cable is pulled, bent, or otherwise subjected to external force.
The optical fiber cables of the loose tube structures illustrated in FIG. 1 and FIG. 2 have superior features, but are poor in terms of the work efficiency at the time of connection.
For example, when pulling out the cores in the middle of an optical fiber cable illustrated in FIG. 1 with a tape core stack 57 accommodated in a pipe-shaped hollow member 56 for connection of cables etc., the pipe-shaped hollow member 56 or jelly-like substance 58 must be cut away or removed. The work efficiency when connecting cables etc. is therefore poor.
Similarly, in the optical fiber cable illustrated in FIG. 2, the jelly 68 must be wiped away when connecting cable. At that time, care must be exercised in the work so as not to scratch or break the tape cores or optical fibers in them. The work consequently takes time.
An object of the present invention is to provide an optical fiber cable having a simple structure which overcomes the above disadvantages in optical fiber cables having tape core stacks accommodated in grooves of a rod-shaped member and therefore can be made thinner and produced at a low cost.
Another object of the present invention is to provide an optical fiber cable which overcomes the above disadvantages in optical fiber cables of the loose tube structure and therefore can be made thinner, can be produced at a low cost, is simple in structure, and is good in work efficiency when pulling out the fiber cores in the middle of the cable when connecting cables etc.
According to the present invention, there is provided an optical fiber cable comprising a tape core stack comprising of a stack of a plurality of tape cores each comprised of an integral assembly of a plurality of optical fibers arranged in parallel and embedded in a plastic, at least one fiber cord member wound around an outer circumference of the tape core stack along its longitudinal direction, and a sheath formed outside of the tape core stack and the fiber cord member wound around the outer circumference of the tape core stack.
If the optical fiber cable is configured to this way, the optical fiber cable can be made thinner, the manufacturing cost does not rise, and the work efficiency when taking out the fiber cores in the middle of the cable becomes better.
Preferably, a plurality of fiber cord members are wound around the outer circumference of the tape core stack along its longitudinal direction in a parallel state by configuring the cable in this way, separation of the tape cores and fiber cord members becomes much easier and the work efficiency when taking out the fiber cores in the middle of the cable becomes even better.
Specifically, the fiber cord members may be formed from continuous filaments of a synthetic resin such as an aromatic polyamide and polyethylene.
More preferably, when a sectional area of a cable interior surrounded by the sheath is S (mm2), a sectional area of one tape core is T (mm2), the number of tape cores configuring the tape core stack is N, and the amount of the fiber cord members used is A (denier), the following relation stands:
100 (denier/mm2) less than A/(Sxe2x88x92NT) 
If configured in this way, the amount of movement of the tape cores can be reliably reduced and an increase of the transmission loss can be prevented.
Still more preferably, the fiber cord member is wound around the outer circumference of the tape core stack along its longitudinal direction while cyclically reversing in direction.
Still more preferably, the fiber cord member is wound around the outer circumference of the tape core stack along its longitudinal direction at a substantially constant pitch. If configured in this way, the tape cores and fiber cord members can be easily separated after removing the sheath by just pulling the fiber cord members down or to the side by hand.
Still more preferably, the tape core stack is twisted at a predetermined pitch in a longitudinal direction of the optical fiber cable. If configured in this way, the tape cores of the tape core stack are constrained and therefore the stack will not fall apart much and an increase of transmission loss can be prevented even if the cable is repeated bent.
Still more preferably, the tape core stack is provided given slack with respect to the sheath. If configured in this way, the stretching of the optical fibers can be adjusted to fall within the guaranteed limit of reliability even if the cable expands or contracts.
Still more preferably, the cable further comprises an overhead support wire arranged in parallel with the tape core stack and made integral with the tape core stack and fiber cord member by the sheath. The overhead support wire increases the strength and facilitates overhead installation.
Still more preferably, the direction of winding of the fiber cord member is made the same as the direction of twist of the tape core stack. If configuring the optical fiber cable in this way, there is less local bending of the tape cores.
Still more preferably, the pitch of winding of the cord is made the same or substantially the same as the pitch of twist of the tape core stack. If configuring the optical fiber cable in this way, there is less local bending of the tape cores.
Alternatively, the direction of winding of the fiber cord member is made opposite to the direction of twist of the tape core stack. If configuring the optical fiber cable in this way, the tape cores can be easily separated from the fiber cord members and taken out during intermediate branching work and therefore the work efficiently is improved.